A Mitochondrion can be called the "power house" of the cell. Most of the cell's ATP, adenosine triphosphate, is produced withing the mitochondria which is a chemical energy that powers the cell. It is also involved with signaling, cellular differentiation, cell death, and the control of the cell cycle and growth.
The mitochondria acts as a energizer that keeps the cell running. think of it as the energizer bunny of a cell.